Olefin-based elastomeric polymers may be produced by the proper copolymerization of ethylene, an .alpha.-olefin and a diene monomer. The most common such elastomers are copolymers of ethylene and propylene (EP elastomers) and terpolymers of ethylene, propylene, and diene, which are generally referred to as EPDMs. While ordinary EP elastomers can be cured through use of curatives such as organic peroxides, for cures using sulfur and sulfur-containing compounds, the presence of a diene is required. Hence, EPDM elastomers find use in numerous cured applications for which the EP copolymers are not suitable. Currently, EPDMs are commonly commercially produced with vanadium compound-organoaluminum catalyst systems.
EPDMs have many properties which make them desirable for applications for which other types of elastomers are not as well suited. EPDMs have outstanding weather and acid resistance, and high and low temperature performance properties. Such properties particularly suit EPDMs as an elastomer for use in hoses, gaskets, belts, bumpers; as blending components for plastics and for tire side walls in the automotive industry; and for roofing applications. Additionally, because of their electrical insulation properties, EPDMs are particularly well suited for use as wire and cable insulation.
Desirably, an EPDM elastomer should have a reasonably fast cure rate and state of cure; hence its diene content should be relatively high, preferably greater than about three weight percent. The cure rate for an EPDM elastomer and the final properties of the cured article depend upon the type of diene incorporated. For example, on a comparable diene weight percent basis, an EPDM produced with 5-ethylidene-2-norbornene (ENB) as the diene will have a faster cure rate in sulfur cures than an EPDM produced with dicyclopentadiene (DCPD), or 1,4-hexadiene (HD), whereas EPDMs with 1-4,hexadiene as the termonomer are known to exhibit good heat resistance. For many commercial applications an EP or EPDM elastomer should also have a low degree of crystallinity, measured by Differential Scanning Calorimetry (DSC) as a heat of fusion of 9 cal/g or less, preferably less than 3 cal/g, according to the technique described herein. For an EPDM material to be useful for most elastomer applications, it should have a weight-average molecular weight of at least about 110,000 or, expressed in terms of the Mooney viscosity (ML.sub.1+8 at 127.degree. C.), at least 10. In many applications it is further desirable that the molecular weight distribution of an EPDM should be characterized by a ratio of weight average molecular weight (Mw) to number average molecular weight (Mn), Mw/Mn, less than 5, preferably less than 3.
The heat of fusion of an EPDM is a commonly-used measure of its degree of crystallinity. This property is important in all practical applications of EPDMs because the degree of crystallinity is correlated with physical properties, such as the tensile strength, and also the processability and tack of the EPDM material. Since, in most commercial uses, elastomers are generally significantly higher in molecular weight than plastics, too much crystallinity makes an EPDM material very difficult to process at ordinary temperatures. Also, although good physical properties are of course desirable (e.g., in applications such as hose and tubing, or wire and cable), again, an excess of crystallinity causes an EPDM material to exhibit high hardness and stiffness and a surface with a "plastic-like" rather than a "rubbery" feel, and poor surface tack.
In most current EPDM production processes, the catalysts used for production of high molecular weight EPDM elastomers are soluble catalysts formed from vanadium compounds such as vanadium tetrachloride, vanadyl trichloride, vanadium acetylacetonate, or a vanadyl trialkoxy compound in conjunction with an organoaluminum compound. The activity of vanadium compound catalysts are generally low, e.g., 80-120 g polymer/mmol V.
In current commercial grades of EPDMs, crystallinity is a function of the ethylene content of the polymer and the catalyst system used for its production. For a given polymer composition, the catalyst system controls the fraction of ethylene units present in long ethylene sequences (long runs of ethylene units), which are capable of crystallizing. On the other hand, when a given catalyst system is used in a given reactor configuration, polymers with higher ethylene content will always have more long ethylene sequences, hence will be more crystalline. For current commercial EPDMs based on vanadium catalysts, the nature of this relationship is such that polymers are completely amorphous (non-crystalline) at ethylene contents below approximately 55 wt % and possess significant crystallinities (i.e., heat of fusion greater than approximately 0.05 cal/g) at ethylene contents of approximately 55 wt % or greater. The degree of crystallinity exhibits less dependence on the diene content of the EPDM material than on the percentage of ethylene. For an EP or EPDM produced by the vanadium catalyst system, VOCl.sub.3 -ethylaluminum sesquichloride for example, a heat of fusion (HOF) of roughly 3 cal/g is obtained at 67 wt % ethylene, while HOF is as high as 9 cal/g at 78 wt % ethylene. The HOF of an EPDM at a given ethylene content may be used to compare the crystallinity of polymers produced by a given catalyst system. In order for the catalyst system to be useful for commercial production of an EPDM elastomer, it is desirable for the crystallinity of the polymers to be roughly comparable to that of currently available commercial grades of EPDM.
Since the recent advent of metallocene-alumoxane coordination catalyst systems for the production of polyethylene and copolymers of ethylene and alpha-olefins (e.g., linear low density polyethylene), some effort has been made to determine the suitability of particular metallocene-alumoxane catalyst systems for the production of EPDM elastomers. For a metallocene-alumoxane catalyst system to be commercially useful for the production of EPDM elastomers, it should produce high yields of EPDM relative to the amount of catalyst in a reasonable polymerization time, and provide for adequate incorporation of a diene monomer, and preferably provide a nearly statistically random distribution of monomers in the polymer chain, while enabling good control of molecular weight over a wide range while yielding a relatively narrow molecular weight distribution.
Two publications have addressed the production of EPDM elastomers by processes using particular metallocene-alumoxane catalyst systems. Kaminsky, J. Poly. Sci., Vol. 23, pp. 2151-64 (1985) reports upon the use of a soluble bis(cyclopentadienyl) zirconium dimethyl-alumoxane catalyst system for toluene solution polymerization of elastomers containing ethylene, propylene, and ENB. Kaminsky employed this catalyst at low zirconium concentrations, high Al:Zr ratios and long reaction times to prepare, in low yields, high molecular weight EPDM elastomers having high ENB incorporation. Although of interest, the method by which Kaminsky reports such EPDM elastomers to be producible with a bis(cyclopentadienyl)zirconium dimethyl-alumoxane catalyst system is not suitable for commercial utilization. In particular, the long induction times required for Kaminsky's catalyst system to reach its full activity, a period on the order of hours without diene present, and longer with diene present, precludes commercial operation wherein such long residence times are economically infeasible.
Similar to Kaminsky, Japanese Kokai 62[1987]-121,711 illustrates the use of a soluble bis cyclopentadienyl) zirconium monohydride monochloride-alumoxane catalyst system for toluene solution polymerization of ethylene and butene-1 wherein, variously, 5-ethylidene-2-norbornene ENB), 5-vinylidene-2-norbornene (VNB), and dicyclopentadiene (DCPD) were employed as the diene. Japanese Kokai 121,711 further suggests, but does not illustrate, that the zirconocene component of the catalyst system may be a bis(indenyl) zirconium hydride or bis(tetrahydroindenyl) zirconium hydride rather than a bis cyclopentadienyl) zirconium hydride. Although Japanese Kokai 121,711 suggests that .alpha.-olefins other than 1-butene can be employed, it illustrates only the production of an ethylene-butene-1-diene elastomer (EBDM) material in a continuous flow atmospheric pressure reaction. Although of interest, the low product yield in view of the high monomer requirements for such process renders it undesirable for commercial utilization.
Although the weather and acid resistance and high and low temperature performance properties of an EPDM elastomer make it a desirable material for a wide variety of high volume elastomer applications, a major factor affecting production costs and hence the utility of an EPDM in these applications is the diene monomer cost. The diene, whether 5-ethylidene-2-norbornene (ENB), 5-vinylidene-2-norbornene (VNB), dicyclopentadiene (DCPD) or 1,4-hexadiene (HD), is a more expensive monomer material than ethylene or propylene. Further, the reactivity of diene monomers with metallocene catalysts described in the prior art is lower than that of ethylene and propylene. Consequently, to achieve the requisite degree of diene incorporation to produce an EPDM with an acceptably fast cure rate, it has been necessary to use a diene monomer concentration which, expressed as a percentage of the total concentration of monomers present, is in substantial excess compared to the percentage of diene desired to be incorporated into the final EPDM product. In turn, the poor conversion of diene monomer increases the cost of production, since the substantial amounts of unreacted diene monomer must be recovered from the polymerization reactor effluent for recycle.
Further adding to the cost of producing an EPDM is the fact that exposure of an olefin polymerization catalyst to a diene, especially the high concentrations of diene monomer required to produce the requisite level of diene incorporation in the final EPDM product, often reduces the rate or activity at which the catalyst will cause polymerization of ethylene and propylene monomers to proceed. Correspondingly, lower throughputs and longer reaction times have been required, compared to the production of an ethylene-propylene copolymer elastomer (EP) or other .alpha.-olefin copolymer elastomer.
To date there has been no suggestion in the art of a process utilizing a metallocene-alumoxane catalyst system which possesses the necessary combination of properties, namely, high activity in presence of diene monomer, high diene incorporation and conversion rate, high polymer molecular weight with high yield, which are requisites for the economical manufacture of a high molecular weight EPDM elastomer product. Nor has there been a suggestion in the art of a metallocene-alumoxane catalyst system which will produce high ethylene content EPDM materials having a heat of fusion value below about 3 cal/g and a desirably narrow molecular weight distribution (MWD) of Mw/Mn less than 3.0.